Displacement pumps, as is well known, are pumps which generate a discontinuous fluid flow, particularly a pulsing fluid flow, in the lumen of a flow vessel deformable at least in sections, particularly elastically, such as a flexible tube. For example, U.S. Pat. Nos. 4,909,710, 5,165,873, 5,173,038, 5,263,830, 5,340,290, 5,683,233, 5,701,646, 5,871,341, and 5,888,052 as well as WO-A 97/41353, WO-A 98/22713 and WO-A 98/31935 each disclose an apparatus for generating and conducting a discontinuous fluid flow which comprises a displacement pump with at least one flow vessel of deformable lumen, which serves to conduct the fluid flow, and with a pump drive for deforming the lumen of the flow vessel.
During operation of the displacement pump, the pump drive acts on sections of the fluid-conducting flow vessel such that displacement motions are imparted to the flow vessel which temporarily deform the lumen of the flow vessel, particularly in an oscillating manner, thus transferring the fluid in the desired direction of flow. In each of the displacement pumps disclosed in U.S. Pat. Nos. 4,909,710, 5,173,038, 5,340,290, 5,701,646, and 5,871,341 and in WO-A 97/41353, peristaltic displacement motions are produced by a non-circular-cylindrical surface of a pump drive rotating about an axle, which surface rests against the flow vessel, while in U.S. Pat. Nos. 5,165,873, 5,263,830, 5,683,233, and 5,888,052 as well as in WO-A 98/31935, the displacement motions are caused by linear motions that a pump drive comprising pumping fingers performs against the flow vessel.
The drive motor for the pump drive is usually an electric motor coupled directly to the pump drive by a drive shaft. The drive motor and the pump drive may also be coupled together by a toothed gearing or a belt drive. Furthermore, an eccentric or cam disk or a crank mechanism, for example, may be used to provide mechanical coupling between the electric motor and the pump drive, see WO-A 98/22713 and U.S. Pat. Nos. 5,165,873, 5,263,830, 5,683,233, and 5,888,052. Instead of an electric motor, a piston-type air motor or a hydraulic motor can be used as the drive motor for producing linear finger motions, as is disclosed in WO-A 98/31935, for example.
Displacement pumps of the kind described, because of a substantially homogeneous, smooth internal wall of the flow vessel and because of the absence of drive elements rotating in the fluid flow, are particularly suited for applications in which stringent chemical and/or biological purity requirements are placed on the fluid-conducting lumen of the flow vessel. Therefore, displacement pumps are frequently used in samplers for chemobiological analyses, particularly in drinking water and sewage treatment plants. Such samplers with a displacement pump are shown in U.S. Pat. Nos. 5,587,926 and 5,701,646, for example.
A physical parameter that is important for the operation of such samplers, particularly for metering liquid samples, is the actual volume of liquid delivered or metered. To determine this volume, the instantaneous volume flow rate of the liquid is determined as a measure of the volume of liquid delivered per unit time, and integrated over a delivery time.
During steady-state operation of the displacement pump, the volume flow rate is strongly dependent on the rate of the displacement motions. This relationship is virtually linear over a wide operating range of the pump, i.e., the volume flow rate is proportional to the rate of the displacement motions, and thus to a set oscillation frequency of the lumen. Therefore, particularly during steady-state operation of the displacement pump, the calculation of the volume of fluid delivered is frequently based on an average volume flow rate for a set displacement motion.
The displacement motions of the flow vessel, and thus the oscillations of the lumen of the vessel, are commonly determined indirectly. To accomplish this, a drive motion of the drive motor is sensed, for example at the motor's drive shaft, using electrodynamic or optical revolution counters, and mapped into a drive signal representative of this drive motion. In suitable evaluation electronics, the drive signal is converted into the volume flow rate and/or into measurement signals representative of the volume of fluid delivered.
However, the drive motions, and thus the measurement signals derived therefrom, are representative of the volume flow rate only if, on the one hand, the flow vessel is filled with liquid in a known manner, particularly completely, and if, on the other hand, no slip occurs between the pump drive and the drive motor. Slip may easily occur in the case of a belt drive or in the case of a pump drive that is merely press-fitted to the drive shaft.
This degree of filling of the flow vessel is strongly dependent on the mounting position of the flow vessel, particularly on the instantaneous suction head. This can be determined a priori, e.g., during start-up, and stored as a setting value in the evaluation electronics, but in the case of samplers, particularly in the case of mobile samplers, the mounting position is variable, i.e., it must be determined anew for each application and, if necessary, stored. Furthermore, the mounting position, particularly also in the case of stationary samplers, may vary because the liquid level at a liquid-sampling location is subject to more or less wide variations.
It has also turned out that, when viewed over the entire operating time, material properties of the flow vessel, such as its tightness, its elasticity, or a property of the inside wall, may also be subject to permanent changes. For instance, deposits on the inside wall may lead to necking or clogging of the flow vessel, and must be detected in time or precluded. Also, damage to the flow vessel, such as leaks, may result in the apparatus becoming useless.
To monitor a displacement pump, particularly a current operational status of the pump drive and/or the flow vessel, additional measures are therefore necessary which detect one or more of the aforementioned parameters during operation and which compensate for the effect of these parameters on the calculated volume flow rate.